The present invention is directed to power control systems for electric traction motor propelled vehicles and, more particularly, to a control system having a plurality of variable current limit functions.
In present day electric vehicles, electronic power regulators are used to control the torque, or speed, developed by electric traction motors. Typically the regulator comprises a time ratio of chopper circuit which varies the power developed by the motors by controlling the percentage of time that the motors are connected directly to a power source. For maximum mobility, the power source is a battery which limits the available power to the motors. The regulator also includes apparatus responsive to accelerator position for varying the ratio of conduction to non-conduction time of the chopper circuit.
The control system for the power regulator includes a current limit circuit which limits the maximum average current which may be supplied to the traction motors. Depending upon the mode of operation, the maximum average current to the motor may be limited because of the commutation limits inherent in the motor itself or because of over heating of the power semiconductor devices which regulate the power to the motor. Although a fixed maximum level of current limit is sometimes employed, it is also known to vary the current limit as a function of the percentage conduction time of the time ratio or chopper circuit.
The application of either a fixed current limit or a current limit which varies only as a function of the percent conduction time of the chopper circuit raises some unusual problems when the control system is controlling a separately excited direct current electric motor in a traction vehicle such as a fork lift truck. In a separately excited motor system, the motor field winding does not provide current limiting for the low impedance armature winding. Accordingly, a very large armature current can be obtained under stall conditions if the solid state switch is bypassed with a contactor as is customary with series motors since armature resistance is typically only one-third of the total motor resistance. If a wheel of the truck drops into a depression or runs up against a dock plate, a very high current may be required from the solid state switch for a short time interval in order to allow the truck to overcome the dock plate or to climb out of the depression. If the regulator is designed to provide this high current capacity continuously, the power handling requirements are large and may require activation of the thermal protection circuit at an unacceptably low value to prevent damage to the chopper circuit as well as a need for unnecessary thermal capacity of commutating circuit components. Alternately, such a high continuous current limit could result in damage to the traction motor if it were misused under normal conditions.
It is an object of the present invention to provide an upward current limit system for a chopper circuit controlled electric traction motor.
It is another object of the present invention to provide a current limit system which permits a current overload without causing motor or control circuit deterioration.